Currently, there is a highly politicized debate on the comparative risks of naturally occurring and synthetic chemicals.

Since humans are often exposed to complex mixtures of both classes of chemicals, the real-world value of this debate is unclear.

However, in any such comparison, it is important to realize that some naturally occurring substances - such as lead or tobacco - have been so altered or disturbed by human activity that they are best considered as products of human activity, or anthropogenic.

Reasons for concern over synthetic and anthropogenic chemicals include :

persistence and propensity for long-term storage in biota and the environment ;